Interpretive Summary: Heat stress during lactation accounts for 10 to 25% of milk production loss of dairy cattle. It is known that cooling of cows during the 60-day period prior to calving (dry period) increases milk production in the ensuing lactation period, but the mechanism of this improvement is unclear. The purpose of this study was to evaluate the effects of heat stress during the dry period on lactation performance and expression of metabolism genes in the liver of dairy cattle. Our results indicated that cooling of cows during the dry period improves lactation performance, possibly through the gene SOCS-2 and its regulation of fat metabolism in the liver.

Technical Abstract:
Environmental factors, especially temperature and light exposure, influence health and productivity of dairy cows during lactation, possibly via similar physiological mechanisms. For example, heat stress is a critical component of lowered milk yield during summer. Yet less is known about the impact of heat stress during the dry period. The objective of the study was to evaluate the effect of heat stress prepartum under controlled photoperiod on lactation performance and hepatic metabolic gene expression of periparturient Holstein cows (n=16). Cows were dried off approximately 46 d before expected calving date and assigned to treatment randomly after blocking by mature equivalent milk production. Treatments consisted of a photoperiod of (14L:10D) and either: 1) Heat stress (HT) or 2) Cooling (CL). Rectal temperature was measured 2X daily during the dry period. After calving, cows were housed in a free stall barn with cooling, and milk yield was recorded daily up to 210 DIM. Daily DMI was measured from -35 to 42 d relative to calving. Liver biopsies were collected at dry off, -20, +2, and +20 d relative to calving for cows on HT (n=5) and CL (n=4) to measure mRNA expression of suppressors of cytokine signaling-2 (SOCS- 2), insulin-like growth factor binding protein -5 (IGFBP5), a key transcription factor in lipid biosynthesis (SREBP1-c), and enzymes of lipid metabolism (FASN, ACACA, and ACADVL) by real-time qPCR. Heat stress increased rectal temperatures (39.2 vs. 38.8oC) and decreased DMI at 0 and 2 wk relative to calving (P < 0.05) and 3.5% FCM postpartum (26.1 vs. 35.4 kg/d) compared with CL cows. Relative to CL cows, hepatic mRNA expression of SOCS2 and IGFBP5 was down-regulated in HT cows. Expression of ACADVL was up-regulated in CL cows at d +2 but down-regulated at d +20 relative to HT cows. These results suggest that heat stress abatement in the dry period improves subsequent lactation, possibly through SOCS-2 and its regulation of hepatic lipid metabolism.